Posts Tagged ‘civil twilight’

Soldiers, mariners, hunters, backpackers, and birders are aware of three dawns each morning. The first is “Astronomical Twilight.” Astronomical twilight, that period each morning when the disc of the sun is between 18 degrees and 12 degrees beneath the horizon, marked by the faintest lightning of the eastern horizon. It is still much too dark for humans to move around without supplemental illumination. Hardly any perceptible light arrives yet. It’s more a feeling than visible light. Around here, it currently begins about 4:30 A.M. and ends a bit after 5:00A.M.

Since the migrating flock of Western Tanagers arrived in our yard and we made our great scientific discovery, our days begin at astronomical twilight. The tanker truck, loaded with grape jelly, arrives then. We pump it off the truck into our two-car garage which is the only place we have large enough to contain it all. (I don’t know if we’ll ever get the garage clean again. Maybe the ants will help.)

“Nautical Twilight” arrives about the same time offloading the grape jelly finishes. Nautical twilight is that portion of the dawn when the disc of the sun is 12 degrees to six degrees below the horizon. Now fewer stars remain visible and the horizon is indistinct but discernible. It remains too dark to work outside safely, although the shapes of large objects are barely visible. Robins begin singing toward the end of nautical twilight. Western Tanagers, whose song is similar to a robin’s wait a bit longer to start their day.

We’re too busy to listen to the robins. All the jelly dishes must be retrieved and cleaned and refilled.

It’s 5:30 A.M.

Now “Civil Twilight” arrives. The sun, six degrees beneath the horizon, begins to illuminate the earth. The horizon is clear and only first magnitude stars and planets remain visible. No longer do we need flashlights to do our work for the tanagers, although flashlights remain handy. Other birds are singing and we have only a few minutes left to get the refilled jelly dishes to their appointed holders before the tanagers stir, demanding breakfast.

By sunrise, the feeders are full and we rush inside for a quick breakfast.

After breakfast we spend all the remaining daylight hours shuttling grape jelly to the feeders. One of us must also fill the hummingbird feeders, the seed feeders, and the tray feeders for our other avian guests. Additionally, photos must be taken and that requires getting close and not moving for long periods of time, which gives us time to remember Huck Finn and Tom Sawyer hiding from Jim:

There was a place on my ankle that got to itching, but I dasn’t scratch it; then my ear began begun to itch; and next my back, right between my shoulders. Seemed like I’d die if I couldn’t scratch. Well, I’ve noticed that thing plenty times since. If you are with the quality, or at a funeral, or trying to go to sleep when you ain’t sleepy [or trying to photograph birds in close] – if you are anywheres where it won’t do for you to scratch, why you will itch all over in upward of a thousand places.

Tanagers cannot live by grape jelly alone. They must have protein. Normally they get that from the bugs they eat, but one reason they are here now is the unusually chilly spring. By now they are usually in their favored habitat which is north of here and higher. They prefer coniferous and mixed coniferous forests full of pinons, juniper, Douglas Fir, and Ponderosa Pine. They also like Aspen groves, as long as the Aspen are high enough. But this cooler than normal spring may have kept them lower for longer. And we don’t have as many bugs yet either, so we supplement their grape jelly with peanuts, peanut butter suet, and orange flavored suet. They prefer the orange suet which we place just beneath the tray where the real oranges sit. Tanagers eat oranges too.

But nature is messy, so Starlings come to eat the suet and the White-winged Doves congregate on the tray feeder which has both jelly dishes and peanuts. They must be shooed away from time to time so the tanagers (and the orioles) can get to the jelly.

We doubt that the tanagers are mating yet. We do not live in their favored habitat and we see fewer females. Western Tanagers are probably monogamous, at least for one breeding season at a time, so more females are needed.

The same three twilights occur again after sunset, in reverse order, although we have no time to attend to sunsets and won’t until the tanagers leave us. We consulted an ornithologist who was in the store yesterday. He predicts that the tanagers will leave in another week or so.

We’re not up-to-date on the latest sleep deprivation research, so if any of you know how long we can go without sleep before psychosis sets in, please write.

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For more on twilight, here are definitions and explanations. For calculations of each where you live, here is the U.S. Navy Observatory page to do that. You can also see the times and the world clock page.

We exaggerated a bit about filling the garage with grape jelly, but not that much. In the last five days, we’ve served twelve pounds (5.5kg) of grape jelly.